Adding my own cent, balls. Big dicks are nice, but you need the balls to match. In the same way that a penis that's long but thin looks ridiculous, a large thick shaft with average balls doesn't work.
Different people have different ratio preferences, I know, but if your protag has a cock the size of their forearm (fist included), then you need balls that match, at least fist-sized I'd say.
Similarly, cum. I know, not everyone is into major excessive cum, but it's just disappointing if your giant dick onlys cums as much or barely more than an average one. This is fiction, have fun with it.
Lastly, as others have said, monster protagonists. Not entirely related to the big cock kink, but it does justifies more extreme sizes and can make for some narrative fun. Just don't make another brainless rape thing with a monster protag, please. Put some thought into it.